Radiation is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, to identify hidden contraband at airports, seaports, and public buildings, for example. The contraband may include hidden guns, knives, explosive devices, illegal drugs, and weapons of mass destruction, such as a nuclear or a “dirty” radioactive bomb, for example. One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. Radiation transmitted through the object is attenuated to varying degrees by the contents. The attenuation of the radiation is a function of the density of the materials through which the radiation beam passes. The transmitted radiation is detected and measured. Radiographic images of the contents of the object may be generated for inspection. The images show the shape, size and varying densities of the contents.
The stationary source of radiation used in a common inspection system is typically a source of X-ray radiation of about 160 KeV to about 450 KeV. The X-ray source may be a source of Bremsstrahlung radiation, for example. The X-ray source in this energy range may be an X-ray tube. X-ray radiation of 450 KeV will not completely penetrate large objects such as cargo containers. Standard cargo containers are typically 20–50 feet long (6.1–15.2 meters), 8 feet high (2.4 meters) and 6–9 feet wide (1.8–2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term “cargo conveyance” encompasses cargo containers and pallets.
While the smuggling of guns, explosives and other contraband onto planes in carry-on bags and in luggage has been a well known, ongoing concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo conveyances. Only 2%–10% of the 17 million cargo containers brought to the United States by boat are inspected. “Checkpoint Terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.
Atomic bombs and “dirty bombs,” which use a conventional explosion to disperse radioactive material over a wide territory, are examples of nuclear devices that may be smuggled in cargo conveyances and smaller objects. Radioactive, fissionable, fissile, and fertile materials that may be used to manufacture atomic devices, may also be similarly smuggled in such objects. Fissile materials, such as uranium-235, uranium-233, and plutonium-239, may undergo fission by the capture of a slow (thermal) neutron. Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. Uranium-238, for example, may be converted into plutonium-239. Thorium-232, for example, may be converted into uranium-233. Fissionable, fissile, and fertile material are referred to herein as “nuclear material.”
Identification of nuclear devices, nuclear materials, and radioactive materials (that may not be nuclear materials), by manual inspection of the contents of an object, such as a cargo conveyance, is too stow for regular use. Identification of radioactive materials and nuclear devices by passive inspection systems, such as a radiation detector, while faster, is difficult. For example, the radiation detector may be positioned along a path of an object. Since nuclear materials are typically dense, however, they absorb most of the photons they emit. Shielding material, such as iron, lead, tungsten, or palladium may also be used to block the escape of radiation, preventing its detection. In addition, certain fissile materials, such as uranium-233, uranium-235, and plutonium-239 while radioactive, have exceedingly long half-lives (on the order of 104–108 years). The count rate from spontaneous decays for such material is so low, that passive detection is not reliable. Also, a-relatively small amount of radioactive material may be located within a large cargo conveyance. It is also difficult to distinguish nuclear devices and nuclear materials from other dense items that may be contained within the object by standard X-ray scanning.
The information that may be derived about the material composition of the contents of objects by X-ray scanning may be enhanced by the use of radiation beams with energy spectra having two different energy endpoints (peak energies) that interact differently with the contents of the object. The interaction is material dependent. For example, two X-ray beams with energy spectra may be provided by X-ray sources with accelerating potentials of 6 MV and 9 MV or higher, which generate X-ray radiation beams with peak energies of 6 MeV and 9 MeV, respectively. For an X-ray beam having a peak energy of 6 MeV, the X-ray radiation will be attenuated mainly by Compton scattering. There is not much pair production over most of that spectrum. For an X-ray beam having a peak energy of 9 MeV or higher, more pair production is induced. Compton scattering also takes place. A ratio of the transmitted radiation detected at two energy endpoints may be indicative of the atomic numbers of the material through which the radiation beam passes. Although pair production starts at 1.022 MeV, Compton scattering predominates until higher peak energies are reached.
U.S. Pat. No. 5,524,133, for example, discloses scanning systems for large objects such as freight in a container or on a vehicle. In one embodiment, two stationary sources of X-ray radiation are provided, each emitting a beam that is collimated into a fan beam. The sources facing adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. In addition, X-ray radiation of two different energies are emitted by each source. One energy is significantly higher than the other. For example, energies of 1 MeV and 5 or 6 MeV may be used. A ratio of the mean number of X-rays detected at each energy endpoint by the detector array as whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby determined.
A further complication with X-ray scanning is that measurements of radiation after interaction with the object under inspection are statistical. The accuracy of a measurement of X-ray radiation transmitted through an object is limited by the number of photons used to make the measurement, as well as intrinsic system noise, for example. Repeated measurements of the same quantity typically yield a cluster of measurement values around a mean value. A plot of the cluster of measurements typically forms a “normal distribution” curve. The dispersion of the individual measurements (the width of the normal distribution curve) is characterized by a standard deviation. Using Poisson statistics in X-ray scanning with a monochromatic X-ray beam, the percentage error of the measurements is one (1) divided by the square root of the number of photons detected, exclusive of system noise. As more photons are detected, the standard deviation decreases and the accuracy of measurement increases. While the number of photons detected may be increased by increasing the scanning time, it is generally not acceptable to slow the throughput rate of a typical X-ray scanning system. For example, it is unacceptable in the current marketplace to significantly delay the passage of cargo conveyances through ports or borders, or to delay the screening of passenger's bags and luggage at airports.
The accuracy of a scanning system seeking to identify a material, such as uranium, for example, may be characterized by its “sensitivity” and its “specificity”. Sensitivity is the probability that the presence of uranium in a cargo conveyance will be identified. A system with high sensitivity will identify more true positives (correct identification of the presence of uranium) and fewer false negatives (missed detection of uranium) than a system with low sensitivity. However, increased sensitivity may result in an increase in the number of false positives, which may not be acceptable. Specificity, which is a statistical measure of accuracy, is the probability that the scanning system will properly identify the absence of uranium in a cargo conveyance, for example. A system with high specificity will identify fewer false positives (identification of uranium in a cargo conveyance when it is not present), than a system with low specificity.
The collection of insufficient photons may result in measurement distributions with large standard deviations. The distributions for materials of interest, such as uranium, may therefore overlap the distributions of other, non-threatening materials. Therefore, it may not be clear whether a particular measurement is indicative of a material of interest or not, resulting in false positives. Practical, efficient, and non-intrusive methods and systems for detection of nuclear materials hidden inside cargo conveyances and other objects, with higher accuracy, are still needed.